Internal combustion engines are in widespread use for various motor vehicles such as automobiles or trucks. Such vehicles usually have an engine cooling system which includes the engine that cooperates with a radiator and a pump for circulating cooling fluid, generally a mixture of water and antifreeze, to and from the engine. A fan is often used for cooperating with the radiator to cool fluid heated by the engine and passing through the radiator during operation of the vehicle.
In these motor vehicles, most heat related damage in and around the engine occurs when the engine is shut down or in an idle position because the cooling fluid no longer circulates through the engine cooling system and often boils behind the combustion chambers. A vehicle water (antifreeze/coolant) pump is often used in such systems for circulating fluid through the engine and radiator of the vehicle. The water pump conventionally has a drive shaft carried by the pump which is driven by the automobile engine via a pulley and a belt which cooperates with the water pump drive shaft and the engine crankshaft.
These water pumps conventionally operate only when the engine is running. The pumping of fluid through the engine cooling system ceases when the engine stops resulting in sharply rising engine block temperatures from the heat being built up in the fluid within the block. There is an excessive temperature increase, particularly in transverse mounted engines, front wheel drive automobiles, and other engines which have high operating temperatures also to reduce hydrocarbon and carbon monoxide emissions. These increased under-the-hood temperatures, in turn, significantly reduce the useful life for rubber and plastic parts in the engine compartment.
Also, although operation of the cooling fan is necessary when the engine is hot, such as occurs when the vehicle is stationary or is moving at low speeds and relatively high temperatures, much of the time operation of the fan occurs when it is not required. When the vehicle engine initially is started, the engine is cold and operation of the cooling fan prior to the time the engine warms up to its operating temperature clearly is not necessary. In addition, when a vehicle is operating at higher speeds, such as ordinarily encountered in highway driving and even in most aspects of city driving, sufficient air passes through the radiator to cool it without the cooling fan operating. This high speed air cooling process is also known as ram air cooling.
With increasing concern for efficiency in motor vehicle operation and, in addition, concern over the high cost of fuel for operating such vehicles, substantial effort has been devoted to improving the number of miles travelled for each gallon of fuel consumed. It is known that the radiator cooling fans require substantial amount of horsepower to rotate them. This is particularly true of large engine motor vehicles such as trucks or racing automobiles which require large radiators and correspondingly large cooling fans. Several horsepower of energy are consumed in the rotation of fans for such vehicles. Thus, the radiator fan is operated or being rotated by the engine when it is not necessary to effect cooling of the engine and a clear waste of engine horsepower results.
Additionally, when an engine is cold, it is not desirable to increase the flow of air through the radiator since this tends to lengthen the time required to heat the cooling fluid up to the desired operating temperature of the engine. Thus, it has been recognized that it is highly desirable to rotate the radiator cooling fan only when the temperature of the engine coolant is hot enough to require the operation of the cooling fan to draw air through the radiator.
In the case of a motor vehicle provided with an air conditioner, a condenser is also often mounted directly in front of the radiator of the engine thereby further restricting engine room ventilation. As a result, when the motor vehicle is forced to travel slowly on a congested urban street or the like, the engine is, and as is well known, overheated to the extent the engine may stall.
This stalling takes place primarily because the addition of the condenser disposed in front of the radiator increases the flow resistance of air passing air therethrough and therefore decreases the flow rate of air for cooling. Because the air temperatures increase when passing through the condenser, which generates heat when condensing the refrigerating medium of the air conditioner, air passing through and around the condenser then has a relatively high temperature when passing through the radiator.
When the engine stalls under such conditions, the temperature of the area surrounding the engine is increasingly raised due to heat generated by the engine itself or the exhaust gas from the fuel system. As a result, a phenomenon such as percolation or vapor lock often takes place and, accordingly, it becomes very difficult to restart the engine.
One known measure to prevent such overheating of the engine when driving at low speeds is to increase the flow rate of cooling air drawn by a fan. Various methods have been heretofore proposed based on this measure. These methods include, for example, a method for increasing the idling speed of the engine and a method of providing an electric motor to drive a fan. The former method; an example of which may be seen in U.S. Pat. No. 3,894,521 entitled "Overheat Preventing Device For Motor Vehicle Engine" by Sakasegawa et al., however, has disadvantages particularly in a vehicle provided with a torque converter that is accompanied with problems of creep and transmission shock and, in addition, the temperature of an exhaust gas purifier increases due to increased amount of engine exhaust during idling. The latter method, an example of which may be seen in U.S. Pat. No. 5,079,488 entitled "Electronically Commutated Motor Driven Apparatus" by Harms et al., also has disadvantages in that a considerable large space is required for provision of an electric motor, and often an additional fan, in the engine room (i.e., under the hood of the vehicle). Also, the positioning of the fan and electric motor often disturbs the smooth introduction of cold air when the motor vehicle is travelling at high speed. Other methods have included additional water pumps and a motor integral with the water pump. These other methods, however, likewise take an additional space in the engine compartment and are often complex and expensive.